1. Field of the Invention
The present invention relates to a speed reducer capable of steadily and smoothly transmitting a driving force from an input shaft to an output shaft after the driving force is provided on the input shaft.
The present invention also relates to an electric retractable rearview mirror equipped with the speed reducer for using in an automobile.
2. Prior Art
Conventionally, an electric retractable rearview mirror is mounted on an automobile and the like such that a driving force provided by a motor can cause a mirror body to be retracted and be returned to an original position for usage.
Above-mentioned rearview mirror is retracted and returned to an original position as a following way. As shown in FIG. 3, a motor 11 is mounted on inside the mirror body. The driving force provided by the motor 11 is transmitted to a shaft 10 (an output shaft) fixed on a base member 22 attached to a vehicle body. Then, the mirror body turns on the shaft 10 since the mirror body is rotatably attached to the shaft 10.
The driving force provided by the motor 11 is transmitted to the shaft 10 via the speed reducer, wherein plural gears are engaged with each other. For example, a speed reducer 31 shown in FIG. 4 can be used as aforementioned speed reducer.
The speed reducer 31 comprises a gear box 32 having a first frame 33 and a second frame 34 combined together, wherein a primary worm 35, a primary wheel worm 37a, a second stage worm 37b, a second stage wheel worm 39a, a spur gear 39b, and a final stage spur gear 40 coupled to output shaft 41 are provided.
However, an input shaft 36 is supported by both input shaft-holding portions. One is an input shaft-holding portion 33a provided in the first frame 33. The other one is an input shaft-holding portion 34a provided in the second frame 34. Accordingly, there had been a case such that the input shaft 36 is slantingly fixed to both of these shaft-holding portions when the first frame 33 and the second frame 34 are combined together. Additionally, when these frames 33 and 34 are combined together, the inconvenience had been arisen as following ways. When an interval between the input shaft-holding portions 33a and 34a is narrower than a length of the input shaft 36, a rotation of the input shaft 36 is suffocated due to being tightly supported. On the other hand, when the interval between input shaft-holding portions 33a and 34a is wider than a length of the input shaft 36, a rotation of the input shaft 36 is waggled due to being loosely supported.
Furthermore, an intermediate shaft 38 is supported by two intermediate shaft-holding portions 34b and 34b provided in the second frame 34. Accordingly, the err caused by incorrectly combining the first frame 33 and the second frame 34 together accurately corresponds to a length moved from a length L (shown in FIG. 2) between each core of the input shaft 36 and the intermediate shaft 38. When said length L between each core of the input shaft 36 and the intermediate shaft 38 is changed from a predetermined length, this has inevitably an influence on an accuracy for engaging the primary worm 35 with the primary wheel worm 37a each other. Accordingly, the driving force can not be steadily and smoothly transmitted. Consequently, a problem had been arisen such that a noise is made or a gear is abnormally worn away.
Sophisticated engaging accuracy is needed when gears are engaged with each other, especially for high-speed rotation. However, when two frames are combined together to form aforementioned speed reducer, there was a limit to improve the accuracy for engaging two gears with each other since it was hard task to dissolve the err caused by incorrectly combining the first frame 33 and the second frame 34 together.
Additionally, above-mentioned inconvenience arises from not only said speed reducer used in an electric retractable rearview mirror but also from the speed reducer having plural frames combined together.
Based on above-mentioned point of view, the object of the present invention is to provide the speed reducer capable of always steadily and smoothly transmitting the driving force regardless of the err caused by incorrectly combining frames to form a gear box. Further, another object of the present invention is to provide the electric retractable rearview mirror capable of always steadily and smoothly being retracted and returned to an original position.